The present invention relates generally to semiconductor circuits, and more particularly to compensating for component value variation in semiconductor circuits.
Semiconductor processes typically result in components having actual values that differ from the nominal design value within a statistical range. For example, a resistor designed to have a nominal resistance of 75 ohms may end up having an actual resistance ranging from 60 ohms to 90 ohms, a xc2x120% variation due to variation in the resistivity (sheet rho) of the semiconductor material on which the resistor is fabricated. When the element value is critical to the operation of the circuit, such variation may adversely affect the performance of the circuit. For example, in higher speed circuits that require a close match between the termination impedance of the circuit and the impedance of the transmission line to which the circuit is coupled for optimum performance, too much variation in the termination impedance due to the manufacturing process may result in unacceptable performance such that some semiconductor wafers are unusable. Thus, there lies a need to be able to provide a better tolerance component value in a semiconductor chip even where the process variation is higher such that normally unusable semiconductor wafers are usable.
The present invention is directed to a termination impedance trimming circuit for controlling the variation in the termination output impedance of a circuit that results from sheet rho resistivity variation in an integrated circuit. As a result of the termination impedance trimming circuit, the actual termination output impedance is maintained within a tighter tolerance range than would otherwise be achieved with a fixed, non-trimmed termination impedance that would otherwise have a larger, unacceptable tolerance range. In one embodiment of the present invention, a reference circuit provides a reference current through an array of sensing resistors having different selected values. The reference current through the resistors controls the trip points of respective comparators coupled to the resistors to provide a control signal to a variable termination circuit. In response to the control signal, the variable termination circuit switches resistance branches in or out of the termination output impedance circuit to provide a selected termination impedance. The voltage drops across the sensing resistors will vary according to the sheet rho (resistivity) variation of the semiconductor so that the termination impedance is selected according to the sheet rho variation. The termination impedance is thereby maintained within a desirable range that is less than the variation in the semiconductor resistivity.